Matrix AC electroluminescent (ACEL) thin film displays are usually fabricated as a multilayer stack comprising a first dielectric layer; a phosphor (e.g., ZnS:Mn) layer; and a second dielectric layer; on a glass substrate with parallel stripes of etched transparent (e.g., indium-tin-oxide or ITO) electrodes or conductors. (The terms "conductor" and "electrode" are used herein interchangably).
Successive dielectric/phosphor/dielectric thin film layers are subsequently deposited to form the heart of the electroluminescent display. Aluminum metal electrodes are finally deposited and etched into parallel stripes orthogonal to the transparent conductor stripes to complete the thin film structure of the ACEL display.
For matrix displays, the front and rear electrode structures are sets of parallel lines, with the front transparent set (columns) orthogonal to the rear set (rows).
The choice of dielectric material plays a significant role in the function and reliability of the ACEL thin film display. Good dielectic constant and breakdown strength are required.
Many fabrication techniques for ACEL displays have been reported, including electron beam, sputtering, thermal evaporation, atomic layer epitaxy, or a combination of these methods.
The goal of preparing a large thin film electroluminescent panel capable of displaying a full page of text or high resolution graphics has been pursued vigorously over the past few years. See for example, M. R. Miller et al., "A Large-Area Electroluminescent Display With MAtrix Addressing for Full Video," SID 86 Digest, pp. 167-170 (1986): M. I. Abdalla et al., "Yield Analysis for Electroluminescent Panel Development," SPIE Vol. 256, Advances in Display Technology V, pp. 83-88 (1985); and L. E. Tannas et al., "ACTFEL Displays," SID 82 Digest, pp. 122-123 (1982).
The most important parameter for assessing a material is the density of electric charge it can hold without breaking down. The charge density at breakdown is given by the product of the static dielectric constant and the breakdown field: EQU Qbd=.epsilon.E.sub.bd
This quantity is thickness dependent. For films in the range of about 200 to 400 nm, charge density at breakdown should exceed about 3 micro coloumbs/square cm. For example, zinc sulfide will luminesce when the charge density reaches about 1.2 micro coloumbs/square cm.
The resolution of the display, i.e., the number of lines that can be charged during a row address period is inversely proportional to the duty cycle, which is about 0.2% for a 512 line display.
At 60 Hz, the time allowed to charge all 512 rows is 16 millisec., so the time allocated for a single row is 0.2% of 16 millisec. or 32 microsec. Refresh rate must be at least 60 Hz to avoid flicker.
Below 1 kHz, luminance is linearly dependent on frequency. At higher frequencies, luminance is limited by phosphor decay time.
The stability of the electrical and optical characteristics of the device is also an area of practical concern.
One major difficulty in fabrication of ACEL displays is the precise thickness control required in depositing the complex EL stack. High rate production is particularly demanding. Film thickness variation is manifested as drive voltage variation across the panel. For practica use, film thickness should be maintained within about 12%.
Nonuniformity in operation can also occur if ITO sheet resistance is too high for the display size and resolution. The magnitude of this problem increases rapidly with display size.
All layers should be as clean as possible with minimum density of defects caused by particulate or pinholes to avoid premature breakdown. Integrity of the electrode system has a critical effect on yield. Electrode deposition procedures should provide smooth rows and columns free of shorts or opens.
Prior to the present discovery, rounded or beveled edges were employed on the ITO layer to improve step coverage, reduce electric field concentration, and to prevent breakdown at the column edges. However, in terms of edge breakdown protection, this method was inadequate. The present invention solves this problem.